This invention uses quantum entanglement to provide a means of communication.
In order to understand this invention it is necessary to first review three basic tenets of quantum theory upon which this invention is based. These are:
1. The Heisenberg uncertainty principle, which states thatΔxΔy≧hwhere Δx and Δy are the uncertainty of simultaneous measurements of two complementary quantum state variables x and y, and h is Planck's constant. In other words, the more precisely you measure one of a pair of complementary quantum state variables, the less precisely it is possible to know the other.
The most common example of the Heisenberg uncertainty principle involves the complementary state variables of position and momentum. If you know a particle's position then you can't know it's momentum, and vice versa. In the present invention we will use a different example: polarization of a photon along two axes that are perpendicular to each other and to the direction of travel of the photon. We will refer to these two axes as H and V for horizontal and vertical, but there is no requirement that these axes have any particular absolute orientation, only that they be at right angles to each other. The Heisenberg uncertainty principle for photon polarization states that if you know the photon's polarization along the H axis then you can't know it for the V axis, and vice versa.
2. The principle of quantum superposition and interference. Particles can exist in multiple quantum states simultaneously, a phenomenon known as quantum superposition. Quantum superposition manifests itself as interference, and consequently demonstrates the wave-like properties of quantum particles.
The most common example of quantum superposition is the well known two-slit experiment first performed by Thomas Young in the early 1800's. A laser illuminates a screen in which two narrow closely-spaced slits have been cut. The light that passes through the slits falls upon a second screen. The result is not two spots of light as one might expect, but an interference pattern consisting of a large number of light and dark stripes, demonstrating the wave-like nature of light.
To this point the experiment is purely classical. If one observes the screen closely with appropriate instrumentation one observes that the illumination of the screen is not constant, but in fact consists of a large number of individual “points” of light (photons) that accumulate and generate the interference pattern over time. If the intensity of the laser beam is low enough it is possible to observe and count individual photons arriving at the screen.
The crucial and startling crux of quantum superposition is this: if any attempt is made to measure which slit an individual photon passed through on the way to the screen then the interference pattern vanishes and is replaced by two spots of light, one behind each slit. This is an easily observed macroscopic effect, and requires no special equipment to reproduce. A laser pointer, some 3×5 index cards, and two pairs of polarized sunglasses suffice. The explanation of this phenomenon according to quantum theory is that the interference is the result of the photons existing in a quantum superposition, being simultaneously at both slits. The photon thus interferes with itself according to wave mechanics. If the actual position of the photon is measured then it no longer exists in a quantum superposition; its state is strictly at one slit or the other. Since the photon is no longer at both slits simultaneously it can no longer interfere with itself.
Note that it is generally not possible to determine whether a given photon has or has not interfered with itself. Interference is an aggregate phenomenon, becoming apparent only when a very large number of particles have arrived at the screen. It is possible to compute precisely how many particles must be observed before one can conclude with confidence that interference is or is not taking place, but the details depend very much on the specific configuration of the experiment. The salient point for the purpose of the present invention is simply that such calculations are possible, and the methods for making those calculations are well known to those skilled in the art.
3.Quantum entanglement. This is arguably the most mysterious of quantum phenomena. It was originally proposed by Einstein, Podolsky and Rosen in the 1935 as a proof that quantum theory must be wrong. In 1965 John Bell proposed a method for testing the idea experimentally. The experiment was actually carried out by Alain Aspect in 1981, with the results showing that Einstein was wrong and entanglement actually does occur. Since then this result has been confirmed by numerous experiments, and is at the heart of the field of quantum computing.
The crux of entanglement is this: it is possible to produce pairs of particles such that measurements of certain of their properties always come out with the same result. On its face this is not such a startling claim. For macroscopic objects it is wholly unremarkable. Consider for example a factory that makes widgets in two colors: red and blue, and in two sizes, large and small, and come in packages of two. Whenever we open a package we find that the widgets are always the same color and the same size. Nothing mysterious here.
To extend the widget analogy to the quantum world we have to imagine that the widgets are extremely fragile, and they disintegrate immediately upon exposure to light. If we want to measure the color or size of one of these exquisitely fragile widgets we have to use special equipment. Furthermore (and this is where is starts to get weird) the color and size of the widgets can change according to the following rules: when you measure the size of a widget, its color can change, and vice versa. So any number of measurements of the size of a widget will yield the same result, but if you then measure the widget's color, and then measure its size again you may (or may not) find that its size has changed from large to small, or vice versa. Likewise, once you measure its size and then go back to measuring color you will find that it's color may have changed from blue to red or vice versa.
It sounds very odd, but this is an accurate description of the behavior of quantum particles. Color and size are analogous to a pair of complementary or orthogonal state variables subject to the Heisenberg uncertainty principle like position and momentum, or polarization along horizontal and vertical axes. When you measure, say, size, then color becomes uncertain. In fact, after a size measurement the quantum widget exists in a quantum superposition of both colors simultaneously, and likewise after a color measurement the widget is simultaneously large and small. Furthermore, and this is crucial to the present invention, because the widget (or particle) is in a quantum superposition of states it will interfere with itself under suitable conditions.
It is now possible to see why entanglement is such a mysterious phenomenon. Until a measurement is actually made the particles exist in a quantum superposition. They don't really have a particular size or color; they are simultaneously both red and blue, both large and small. It is only when a measurement is made that the widget/particle somehow “decides” which size or color to become. And somehow, in a way that is not understood, for a pair of entangled particles, when one member of the pair makes such a decision, its counterpart will simultaneously make the same decision at the same time even if the two particles are far apart. (Einstein called this “spooky action at a distance”, and it was his main objection to quantum theory. This “spooky action at a distance” is real, and it has already found application in cryptography, where it is used for the secure distribution of keys.)
It is tempting to try to use this phenomenon to communicate information, but it is not as straightforward as it might seem. In fact, it can be proven that it is impossible to communicate information using quantum entanglement of a single pair of particles. It is tempting to conclude that it is therefore impossible to communicate information using quantum entanglement using more than one particle since if a single particle communicates zero information then N particles could only communicate N times zero information, which is to say, zero information. But, as the present invention will show, this is not the case. The proof of impossibility has a loophole.
To understand the loophole it is necessary to summarize the proof. If you make a measurement on a particle that is a member of an entangled pair then you gain information about its counterpart. In particular, you know what the result of a particular measurement will be (or was). But no information has actually been transmitted between the two particles. It is possible to prove this in a mathematically rigorous way, and the result extends to multiple particles as well.
But (and this is the loophole) making a measurement is not the only thing you can do to a particle. Rather than make a measurement, you can arrange for the particle to interfere with itself instead. For a single particle this also yields no useful information, since a single particle is not enough to reveal the presence or absence of interference. But with multiple particles it is possible to transmit information using quantum entanglement together with interference.
We now proceed to describe how this is done.